Inspired by some posts on Reddit by other users who have done the same thing I modified two bags / backpacks with LED string fairy lights so I could feel part of the magic at Disney World.

Supplies

Backpag / Bag to be customised - I purchased two from Amazon although they're also available on eBay, lots of options appear if you search for Reflective / Luminous / Iridescent / Holographic Geometric Backpack. Links for the ones I bought are here and here
String LED Fairy Lights - These come in a number of lengths / with various different number of LEDs, best to find roughly the right number for the holes /positions on the bag you're using. I bought these, these and these.
USB Battery Power Bank - I purchased a smaller one than I already had to make the project a bit lighter - 3000mAh, unsure how long the battery will last with the LEDs on at this stage but I'm sure it would be a long time!
Wood burning / Pyrography tool - I used one to make the holes in the bag on the advice of Reddit and must say it worked really well - none of the holes frayed or ripped. I went for this one.
Clear Silicone Sealant and Sealant Gun - In order to protect the LEDs and bag from water damage as best as possible I covered all the LEDs from the outside with a blob of Silicone.
Duct Tape - For keeping the LEDs in place inside the bag, I used Gorilla tape although standard duct tape should be fine.

Step 1: Prepare the Backpack

Before getting to the fun part of burning holes and fitting LEDs there is a tedious step - Separate the lining of the front of the backpack.

I used a stitch removal tool to unpick some of the thread holding the layers of the backpack together - in my case this was the front panel, inside panel, zip and a strip of material that held these together.

I tried to remove as little as possible although found that in order to get to the holes at the bottom of the bag it's best to give yourself as much space and maneuverability as possible.

Step 2: Burn Holes

Pretty self explanatory - pick how many holes and where you want them to be and start burning holes!

Best to do this in a well ventilated / outdoors area as the smell from burning material was not pleasant!

Be careful not to touch anything unintentionally with the burning tool - people, surrounding things but also the zip of the bag and any other layers! I caught one part of the zip and whilst it still works fine I can certainly tell.

My burning tool came with a number of different attachments although I found that just using one and wiggling it around was fine - I did a best guess in terms of hole sizing and was restricted by the geometric shapes on the front of the bag anyway.

I also burned holes inside the bag to provide a route through for the wires and buttons to avoid fraying material inside the bag.

Step 3: Secure LEDs

Possibly one of the biggest challenges of the project

I aligned all the LEDs to the rows / lines guided by the geometric shapes, starting in the bottom corner - This makes one of the better patterns when the LEDs are rotating through colours.

I found that using smaller strips of tape and securing the LEDs one at a time was the only way I could get through it - otherwise tape sticks to everything and itself and the LEDs come out of place.

Depending on the size of the holes I managed to push the LEDs through them and they'd kind of hold slightly but still needed a good bit of tape to make sure

Step 4: Test LED Pattern

Because we're still able to remove the LEDs it's best to test that you're happy with the layout of everything - when the LEDs are cycling through colours are you satisfied with the order in which things change and how it looks?

Step 5: Silicone Everything

After all the LEDs are in place and I was happy with things I started adding dabs of silicone over every LED, Another tedious part and not as easy as it should have been.

I'm not particularly happy with all of the dabs and some look rough and either too big or too small

Consistency and an even smooth finish seemed to be impossible for me at least

While the silicone was wet I went around with cocktail sticks to try and tidy up as best as I could - Twirling around the stringy bits and removing it from places I didn't want it to land.

Step 6: Re-stitch the Lining

Hopefully you've remembered how it all looked along the zip before you ripped it apart!

I opted for manually stitching back together the two panels and zip - using standard thread and pins to hold in place as I went along.

Then I went back along with a sewing machine to put the bit of material holding them together.

All went well until the second corner and then I realised things felt a bit tight - be careful how much slack you're giving yourself when starting to stitch.